1. Field of the Invention
The present invention relates to impeller pumps of a type known as Westco pumps, generative or friction pumps, cascade pumps and circumferential-flow pumps that have rotary impellers.
2. Description of the Related Art
A known generative or friction pump 101 for pumping fuel is shown in FIGS. 10(A) and 10(B) and includes an impeller 102. The impeller 102 is disposed within a pump casing 106 and is rotatably driven by a motor 105 via a drive shaft 105a of the motor 105. Grooves 102a are defined in each of upper and lower surfaces of the impeller 102 and are arranged in the circumferential direction of the impeller 102. In order to minimize the outer diameter of the pump 101, a suction port 101a is defined in the pump casing 106, such that the fuel flows from the lower side of the impeller 102 into a first main flow channel 103a defined in the pump casing 106 in a direction perpendicular to the surface of the impeller 102. A second main flow channel 103b is defined in the pump casing 106 on the side opposite to the first main flow channel 103a. A plurality of communication holes 102b are defined in the impeller 102 and each of the communication holes 102b communicates between two of the grooves 102a that oppose to each other in the axial direction of the impeller 102 and defined in the upper and lower surfaces of the impeller 102, respectively. As a result, the fuel may flow uniformly into the first and second main flow channels 103a and 103b disposed on both sides of the impeller 102, so that the pumping efficiency may be improved. This type of known generative or friction fuel pump is disclosed in Japanese Laid-Open Patent Publication No. 5-18388.
However, when the grooves 102a with the communication holes 102b are positioned to communicate with the suction hole 101a during the rotation of the impeller 102, the grooves 102a cannot produce circulation (swirl) of flow of the fuel. Therefore, it is not possible to start the pumping action immediately after the fuel has been drawn into the suction hole 101a. Due to this phenomenon, there still exiests a problem that the pumping efficiency is degraded. In order to improve the pumping operation, the rotational speed of the impeller 102 may be increased. However, this may increase the amount of consumption of the electric power.